The present invention relates generally to Code Division Multiple Access (CDMA) communication systems. More specifically, the present invention relates to a CDMA receiver.
CDMA systems use spread spectrum techniques and multi-code operation to provide higher network capacity in a given bandwidth than a single-code system. The increased capacity may be directed to a single user or shared among multiple users.
To implement a receiver, a CDMA system generally requires knowledge of the identity of the codes used to construct the transmitted signal. The receiver in a User Equipment (UE) may know the identity of all codes, a subset of codes or none of the codes used for any given transmission. Codes associated with signals directed to the desired UE will be referred to hereinafter as “own-UE codes”; and codes associated with signals directed to other receivers will be referred to hereinafter as “other-UE codes.” Typically, a system includes a means in the receiver to know or learn the identity of own-UE codes via initial programming, signaling, acquisition methods or various other techniques that may include trial and error, which may be inefficient from a power or performance standpoint. Systems may or may not provide a means to learn the identity of other-UE codes. Specific codes used to transmit data can be static or can change from time-to-time.
Demodulation of data associated with any one code is subject to degraded bit error rates (BER) caused by interfering own-UE codes and/or other-UE codes. The receiver can benefit from knowledge of the identity of own-UE or other-UE codes by implementing improved methods that provide a lower BER at a given signal-to-noise ratio in a radio channel with a certain multipath characteristic.
Multi-user detection (MUD) is one example of a receiver method that simultaneously processes received signals associated with multiple codes in an attempt to minimize the impact of interference and provide a lower BER, or the same BER in less favorable signal-to-noise-ratio (SNR) or multipath. MUD operates optimally when it is configured for the exact set of transmitted codes. To achieve this, MUD requires knowledge of the identity of transmitted own-UE codes and transmitted other-UE codes. In addition, MUD generally requires an estimate of the transmission channel over which the signal was sent. The estimate of the transmission channel is called the “channel response” or “channel estimate.” The transmission channel may be the same for all codes. If transmit diversity, antenna beam steering or other methods of signaling diversity are used at the transmitter, then different transmission channels may be associated with different codes.
One approach to implementing MUD is to configure the receiver for all codes that may or may not have been transmitted. There are two disadvantages that render this approach undesirable, and possibly impractical. First, the more codes a MUD device is configured to process, the greater the number of computations that are required to demodulate the transmitted data. Hence, configuring a receiver only for codes that have been transmitted requires less power and less processing time. Second, the BER is often degraded if the MUD is configured to process a relatively large number of codes. Hence, configuring a receiver only for codes that have been transmitted generally provides an improvement in BER.
In timeslotted CDMA systems that employ MUD, for example, in 3GPP TDD systems, one or more channelization codes in one or more timeslots are allocated to coded composite transport channels (CCTrCHs). In each timeslot, multiple CCTrCHs may be transmitted and may be directed to one or more UEs.
Each transmitted code is associated with a midamble code shift that may or may not be shared with other transmitted codes. The UE estimates the channel response by processing received midambles. The association between midamble code shift and transmitted codes is not explicit since detection of a particular midamble code shift does not guarantee that an associated code has been transmitted. However, in the specific case of Kcell=16 in TDD, the association is unique.
As one example, during call set-up, a CCTrCH is provided with an allocation of channelization codes and timeslots and these are signaled to the UE. Therefore, the UE has a list of allocated codes. However, since not all allocated codes are used in every transmission, the UE has partial information, (i.e. the information regarding own-UE codes). The list of other-UE codes is not available, except in certain cases where some hint as to the total number of codes is indicated through physical layer signaling.
Each transmitted code is a combination of a channelization code, a channelization code specific multiplier and a scrambling code. The scrambling code is signaled to the UE and the code specific multipliers are associated with channelization codes, so the identity of the channelization code itself is the only one of the three that needs to be determined.
If a code allocated to a CCTrCH is not transmitted, then the CCTrCH is in discontinuous transmission (DTX). A CCTrCH is said to be in “partial DTX” if not all of the allocated codes are transmitted in a given timeslot. It is said to be in “full DTX” if none of the allocated codes are transmitted in a frame.
The identity of the transmitted codes for an entire frame can be derived from the Transport Format Combination Index (TFCI) that is signaled to the UE and is multiplexed with the data signal. The TFCI is transmitted in the first timeslot allocated to a CCTrCH, and optionally in subsequent timeslots in the same frame. Each UE can process the received TFCI to determine the transmitted own-UE codes in each timeslot of the frame. However, this requires demodulating received data symbols and executing various other processes to decode and interpret the TFCI information. In certain receiver implementations, the inherent latency of these processes could result in the identity of transmitted own-codes not being available when received data in the first timeslot (and possibly some subsequent timeslots) in the frame are processed in a MUD-device.
Accordingly, there exists a need for an improved receiver which has the ability to more effectively identify the incoming channels.